PAS as represented by polyphenylene sulfide (hereinafter sometimes abbreviated as “PPS”) is an engineering plastic which is excellent in properties, such as heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical properties, and size stability. PAS can be formed into various shaped products, film, sheet, fiber, etc., by ordinary melt-forming processes, such as extrusion, injection molding, and compression molding, and is therefore widely used in versatile fields, inclusive of electrical and electronic industries and car industries. Further, powdery PAS may also be used in the fields of, e.g., coatings on metals and other materials.
A typical process for producing PAS may comprise a step of reacting an aromatic dihalide compound and an alkaline metal compound in a polar organic solvent such as an organic amide under heating for polymerization (and condensation) (hereinafter referred to as a “reaction step” or “polymerization step”), and steps disposed before and after the reaction step. Principal steps disposed after the reaction step may include: a step of cooling the reaction mixture including the produced PAS to or below a temperature at which the reaction mixture exhibits a vapor pressure below the atmospheric pressure (hereinafter referred to as a “cooling step”) and a step of isolating PAS from the cooled mixture including PAS (hereinafter referred to as a “washing-filtration-drying step”).
In some cases, a dehydration step for adjusting moisture in the system is placed prior to the (polymerization) reaction step that is performed by adding the aromatic dihalide compound. More specifically, in this case, PAS is produced through a combination of respective steps including the dehydration step, the polymerization step, the cooling step, the washing-filtration-drying step, a step of recovering unreacted starting materials, and a step of recovering the polar organic solvent.
Regarding the cooing step, Japanese Laid-Open Patent Application (JP-A) 59-49232 discloses a process for producing polyarylene sulfide wherein the system after the polymerization is gradually cooled at a rate slower than 50° C./min. from the polymerization temperature down to a temperature of 240° C. or below, thereby precipitating the polymer particles and providing fine polymer crystals having sizes of at least 60 Å. The JP reference however includes various descriptions regarding the gradual cooling temperature and the gradual cooling rate, such as down to 240° C. or below, preferably down to at least 200° C. or below for the gradual cooling temperature region, and a rate slower than 50° C./min., preferably a rate slower than 10° C./min. for the gradual cooling rate. Particularly, in the working examples, there are disclosed a mode of gradual cooling at a rate of 1° C./min. from 260° C. down to 150° C. (Example 1) and a mode of gradual cooling at a rate of 0.5° C./min. from 260° C. down to 100° C. (Example 2). Accordingly, these Examples require cooling periods of 110 min. and 220 min. respectively for the cooling from 260° C. down to 150° C.
JP-A 2001-89569 discloses a process for producing PAS particles of high purity through a short polymerization process time by controlling the cooling rate within a range of 0.2–1.3° C./min. in a temperature range of 245° C. or below, based on a knowledge that the formation of polymer particles and the amount of by-products in the resultant polymer particles are affected by the mol ratio of the aromatic polyhalide compound to the polar organic solvent and the cooling rate after the polymerization reaction. The JP reference describes only the reduction in amount of the resultant by-products generated as gases when the product PAS is heated, as the effect of the gradual cooling, and does not refer at all to any other properties of PAS. Further, as for the cooling temperature range, Example 1 refers to 245° C. to 218° C.; Example 2, 245° C. to 212° C.; and Example 3, 245° C. to 210° C. This means that if the temperature range of 245° C. to 218° C. is subjected to gradual cooling at the upper limit cooling rate of 1.3° C./min., the gradual cooling requires a period of at least 20 min., and the shortening of the polymerization cycle including the cooling step is not yet at a satisfactory level.